Endocrine Disruptor 

Screening Program (EDSP)

Weight of Evidence (WoE) 

Concluding Remarks

Thomas M. Steeger, Ph.D.

Environmental Fate and Effects Division

Office of Pesticide Programs

Office of Chemical Safety and Pollution Prevention

U.S. Environmental Protection Agency
Structure Activity 

Relationships

In vitro Studies In vivo Studies

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Population 

Studies/Models

 How responses are integrated from diverse studies

 How the strength of evidence is determined by 

evaluating varying degrees of complementarity and 

redundancy in responses

 How OSRI data are used to further characterize Tier 1 

findings

Application of Adverse Outcome Pathway 

Concept Demonstrates:
Case Study A — Androgen Antagonism

Intact HPG

Tier 1 Assays

Rat Pubertal & OSRI - anti-androgenic responses 

were not observed at a higher level of biological 

organization presumably due to compensatory 

mechanisms.

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Chemical A

 Androgen Antagonism

 An example of a compound that has potential to initiate a pathway but interaction is 

not robustly expressed in organisms with an intact HPG

 An example of FSTRA decision logic —

• Only 2 endpoints impacted that lack specificity (GSI and gonadal histology) in 

absence of overt toxicity

• Responses coincident with overt toxicity at higher doses may be nonendocrine

and reflective of generalized toxicity

 Absence of robust complementarity and redundancy in responses do 

not support potential interactions with estrogen signaling pathway

 Equivocal in vitro; negative mammalian; FSTRA lacks dose response and 

complementarity in responses

 Lack of robust complementarity and redundancy to support HPT axis 

interaction

 AMA negative; marginal responses in T4

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Case Study S — E and A Pathway Interactions

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Tier 1 Assays

OSRI- no effects 

found

Estrogen Pathway

Androgen Pathway
Chemical S

Limited complementarity and redundancy in 

responses support potential interactions with 

estrogen signaling pathway

Lack of robust complementarity and redundancy 

to support HPT axis interaction

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Case Study J — Androgen Antagonism

(altered steroidogenesis) 

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Tier 1 Assays

Weak AR binding and Negative Hershberger does not support AR binding as the MIE

OSRI- effects consistent 

with Tier 1 assays
Chemical J

 Androgen Antagonism

 An example of a compound that has potential to initiate a pathway and robustly 

express responses at higher levels of biological organization in organisms with an 

intact HPG

 Robust complementarity and redundancy

• FSTRA and male pubertal responses diagnostic of A interaction

 Absence of robust complementarity and redundancy in responses do 

not support potential interactions with estrogen binding signaling 

pathway

 ER binding and ERTA in vitro negative

 Female mammalian in vivo negative; however open literature discusses observed 

differences between rats and fish

 Lack of robust complementarity and redundancy to support HPT axis 

interaction

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Case Study N — Androgen Antagonism 

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Tier 1 Assays

* No effect on receptor mediated signaling OSRI - no relevant 

effects
Chemical N

Androgen Antagonism

 MIE is unclear but has potential to initiate a pathway with responses 

expressed in organisms with an intact HPG near limit doses/concentrations

• limited complementarity and redundancy within in vivo assays

Absence of robust complementarity and redundancy in 

responses do not support potential interactions with 

estrogen binding signaling pathway

 FSTRA lacks dose response and complementarity in responses

Negative responses (AMA: Rat) do not support HPT axis 

interaction

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Case Study X — Estrogen Antagonism

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Tier 1 Assays
Chemical X

Androgen Antagonism

 AR binding and ARTA are likely MIE but it is unclear if Chemical X has 

potential to initiate a pathway in organisms. Discrimination between 

estrogen agonist and androgen antagonist in organisms with an intact HPG 

is difficult.

• Negative Hershberger assay, limited androgen-specific effect in vivo

assays

Estrogen Agonism

 A high degree of complementarity and redundancy in estrogenic responses 

strongly support potential interactions with estrogen signaling pathway

HPT Interaction

 Alterations in T4 and TSH in mammals suggest possible HPT axis 

interaction, however, no AMA analogue

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Adverse Outcome Pathways (AOPs)

and the Weight of Evidence (WoE)

Competing AOPs

Endocrine versus non-endocrine pathways or 

mechanisms of pesticidal mode of action that lead to 

overt toxicity

Integrate understanding of overt toxicity

• Chemical A — cholinergic intoxication due to cholinesterase 

inhibition

• Chemical S — uncoupling of mitochondrial oxidative 

phosphorylation and depletion of ATP; irritation that compromises 

the integrity of the gastro-intestinal tract

• Chemical N — nonpolar narcosis

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In Summary

Approach articulated in 2011 EDSP WoE guidance

Promotes systematic use of data

Focuses on what is known about underlying biology as 

described in the AOP as an aid in the WoE process

Understanding degree of complementarity and 

redundancy in observed responses, as well as,

the conditions under which responses occur provides 

basis for judging biological plausibility

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In Summary

The Agency is seeking comments from the SAP on the 

implementation of the basic interpretive process of the 

WoE guidance as illustrated with five case studies

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Next Steps

Conduct a systematic literature review for public 

literature on E, A and T

Integrate different lines of scientific evidence 

using the Adverse Outcome Pathway

Finalize weight of evidence assessments for all 

List 1 chemicals, considering the 

recommendations made by SAP

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